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If your favorite driving companion is a radar detector, you know how false alarms destroy the reliability of your warnings. Two of the five utes here are reckless alarmers, and another does its best to keep you from getting any warning at all.

Audi has an option it calls "side assist." The purpose? To warn motorists too lazy to swivel their necks that another vehicle might be in an adjoining lane.

Never mind Audi's "never follow" slogan; it has followed Wal-Mart et al. by choosing the same cheap-tech proximity sensor used by stores to open doors as customers approach. The Q7 has what amounts to a K-band door watcher in each rear corner.

K-band is a microwave frequency commonly used for traffic radar. The result: This Audi reads like a radar trap to nearby detectors. You can switch off side assist, but the transmitters restart automatically. Why? Audi says it keeps them always on to avoid the "3-to-10-minute warm-up for ideal operation." Another microwave-spewing option, Audi's adaptive cruise control, transmits radar from the front.

The QX56 had the adaptive-cruise option, but Infiniti uses laser instead of radar. Just as bad; it sets off laser detectors, even those within as the laser reflects off signs, trucks, vans, and the like back through its own windshield. You can switch off the cruise, but the laser keeps transmitting as part of another long-shot safety feature called preview braking. If the laser distance sensor thinks you're closing too fast on another vehicle, it doesn't do anything useful like apply the brakes. Instead, it pressurizes the brake-assist feature so that even those weak of leg can get full braking if they wake up and put a foot on the pedal.

While driving the Benz, we noticed greatly diminished radar range, which instantly returned to normal when we held our Valentine One out the side window. Mercedes spokeswoman Michelle Murad says all GLs with the optional three-zone climate control, included on the test car, have "infrared glass," which has a metallic film to reduce solar heating of the interior. The film also blocks radar and laser signals.

Murad says there's an E-ZPass opening in the film up behind the mirror. We have tested detectors through such openings in the past, with bleak results. Our advice for detector users — avoid cars with "infrared glass." — PB